The present invention relates to a hydraulic power transmission suitable to heavy vehicles with diesel engines for construction such as wheel loader, mobile crane, dump truck or the like.
Conventionally, such heavy vehicles have employed only a torque converter as a hydraulic power transmission. The output of the torque converter is connected to a hydraulic speed change gear mechanism including gears for some (usually three or more) speed ranges and a hydraulic clutch for selecting the gears.
However, while the torque converter has a large torque capacity at a low rotation speed ratio of the output to the input, the torque capacity thereof remarkably decreases at high rotation speed ratios, so that the engine torque can not be utilized efficiently, and thus, the rate of fuel consumption deteriorates. In order to avoid problems of the type described above, it has been necessary to increase the size of the torque converter to some extent for increasing the torque capacity.
Further, in said prior vehicles, there is a fear that an engine brake can not sufficiently act on a downward slope of a public road.
Moreover, said mechanism having a plurality of speed ranges has a complicated structure and requires a complicated shifting operation.